BEHAVIOR OF CYCLIC FATIGUE CRACKS IN MONOLITHIC SILICON-NITRIDE

Cyclic fatigue-crack propagation behavior in monolithic silicon nitrid e is characterized in light of current fatigue-crack growth models for ceramics toughened by grain-bridging mechanisms, with specific emphas is on the role of load ratio. Such models are based on diminished crac k-tip shielding in the crack wake under cyclic loads due to frictional -wear degradation of the grain-bridging zone, The notion of cyclic cra ck growth promoted by diminished shielding is seen to be consistent wi th measured (long-crack) growth rates, fractography, in situ crack-pro file analyses, and measurements of back-face strain compliance, Growth rates are found to display a much larger dependence on the maximum ap plied stress intensity, K-max, than on the applied stress-intensity ra nge, Delta K, with behavior described by the relationship da/dN propor tional to K(max)(29)Delta K-1.3. Fatigue thresholds similarly exhibit a marked dependence on the load ratio, R = K-min/K-max; such effects a re shown to be inconsistent with traditional models of fatigue-crack c losure, In particular, when characterized in terms of K-max, growth ra tes below similar to 10(-9) m/cycle exhibit an inverse dependence on l oad ratio, an observation which is consistent with the grain-bridging phenomenon; specifically, with increasing R, the sliding disance betwe en the grain bridges is decreased, leading to less frictional wear, an d hence less degradation in shielding, per loading cycle. The microstr uctural origins of such behavior are discussed.